The invention relates to an apparatus as defined in the opening part of claim 1.
Such an apparatus is commercially available under the type designations AEG9080 and AEG9082 and is consequently known. In addition, reference is made to the patent document WO 97/47117 A1 for such an apparatus. In the known apparatus, the holder compartment, referred to as the back chamber in the patent document WO 97/47117 A1, accommodates only one electroacoustic transducer for the generation of sound, as a result of which only a comparatively low maximum sound volume is attainable with the known apparatus, which is unfavorable particularly in the off-ear mode. Moreover, with respect to the known apparatus it is to be noted that the holder compartment is bounded by a rear bounding wall which is remote from the electroacoustic transducer and which has a plurality of rear apertures covered with an acoustic friction material. These means have been provided in order to obtain an optimum frequency response for the generated sound in the on-ear mode. However, these rear apertures in the rear bounding wall of the holder compartment also lead to an undesirable reduction of the attainable sound volume in the off-ear mode because the opposite-phase sound waves emitted via the rear apertures distinctly attenuate the emitted useful sound waves. Moreover, it is to be noted that in the known apparatus the frequency response for the emitted useful sound waves can be influenced only with the aid of the single electroacoustic transducer, which may impose restrictions as regards a desired frequency response.
It is an object of the invention to preclude the aforementioned problems and restrictions in a simple manner and to provide an improved apparatus for the emission of sound.
According to the invention, in order to achieve the afore-mentioned object with an apparatus as defined in the opening part of claim 1, the characteristic features defined in the characterizing part of claim 1 have been provided.
As a result of the provision of the characteristic features in accordance with the invention it is achieved in a comparatively simple and also comparatively cheap manner that as a result of the opposite-phase drive of the two transducers a satisfactory, i.e. as constant as possible, frequency response is obtained for the useful sound waves emitted into the ear. Moreover, the opposite-phase drive of the two transducers provides a high degree of privacy in the on-ear mode because owing to the opposite-phase drive of the two transducers the sound waves then generated, which are emitted into the acoustic free space via the intermediate space between the bounding wall and the outer wall and the at least one external additional sound port which issues from the intermediate space, propagate into the acoustic free space with only a comparatively low sound pressure, as a result of which these sound waves, after they have emerged into the acoustic free space, are rapidly attenuated to such an extent that, even at a comparatively short distance from the sound-emitting apparatus, the emitted sound, for example reproduced speech, is no longer intelligible to someone who happens to listen in. Moreover, in the apparatus in accordance with the invention it is achieved in a simple manner and without any further separate means that when the apparatus is held against the ear though the apparatus is in the off-ear mode no excessive sound pressure is produced in the user""s ear and the possibility of a hearing defect is thereby precluded. Furthermore, it is thus achieved that, apart from the at least one internal sound port and the at least one additional internal sound port, the holder compartment can be constructed so as to be acoustically impervious, i.e. has no rear ports as in the case of the known apparatus described in the introduction, so that neither any problems caused by such rear ports cannot arise.
An apparatus in accordance with the invention may have internal sound ports in the form of bores which extend, for example, transversely to the bounding wall and whose ends which are remote from the second transducer terminate in the intermediate space. However, it has proved to be very advantageous when an apparatus in accordance with the invention in addition has the characteristic features defined in claim 2. Such a construction has the advantage of lower losses, as a result of which a higher volume of sound is attainable in the off-ear mode.
With an apparatus in accordance with the invention it has further proved to be very advantageous when, in addition, the characteristic features defined in claim 3 are provided. As a result of this construction, which enables the relative position of the at least one external sound port with respect to the two transducers to be chosen as required, the volume of sound produced in an ear in the on-ear mode can be adapted easily and simply to a desired situation.
In an apparatus in accordance with the invention it has further proved to be very advantageous when, in addition, the characteristic feature defined in claim 4 is provided. In this way it comparatively simple to realize a desired overall frequency response.
The afore-mentioned as well as further aspects of the invention will become apparent from the embodiment described hereinafter by way of example and will be elucidated with reference to this embodiment.